I moved my Digikam database from the local SQLite file to a networked MariaDB server. This was mostly fine but Digikam (8.8.0 version) was performing terribly after any face tagging operation.
I would get frequent Force Quit or Wait? questions from the OS, and the UI would hang for 5-10 seconds. This was always matched with a face scan job kicking off and showing up in the jobs queue. Everything generally worked but was incredibly tedious. ~20,000 images in my collection, BTW.
A few times I went through the DB's config and optimized some things, which helped a little. Typical tuning things like increasing memory and threads, and bumping up max_allowed_packet as recommended. Still the issue persisted.
Eventually I learned that there is a Enable background face recognition scan setting. It seems obvious in retrospect, but this completely fixed the issue. I will just need to periodically rescan for faces now to apply what training has been done since the last run.